Yes and no.
Yes you can install OFW over CFW (4.76-O4.76, 4.76->O3.55) but once you upgrade to any OFW > 3.55 you'd need a flasher to downgrade.
So OFW won't let you downgrade ever, and CFW will let you downgrade but only with QA flag. I get it now.
I have a handful more questions though.
1. Aside from something stupid like installing 3.56+ OFW, what should I avoid to make sure I don't brick or become unable to install CFW on my PS3?
2. Is there any way to get my PS3 to output a 480p signal in gameplay? It's a PAL unit and the settings will allow 576p but not 480p. If it's related to the region setting of the console, would a REX release of Rebug 4.76 be able to enable this output? If so, how long would I expect to wait for a REX release?
3. Is OtherOS++ good for anything? Seems like a cool curiosity worth tinkering with.